This invention relates to synthetic fuels and, more particularly, to producing and dedusting shale oil.
Researchers have renewed their efforts to find alternate sources of energy and hydrocarbons in view of past rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal, and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce duce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be efficiently converted to oil by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane, and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam, and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Syncrude is the upgraded product of shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in in situ retorts underground. In principle, the retorting of shale and other hydrocarbon-containing materials, such as coal and tar sands, comprises heating the solid hydrocarbon-containing material to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in vessels where it can be contacted with a hot solid heat carrier material, such as hot spent shale, ceramic balls, metal balls, or sand or a gaseous heat carrier material, such as light hydrocarbon gases, for heat transfer. The resulting high temperatures cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material can be removed from the retort and recycled as heat carrier material or discarded. The combustion gases are dedusted in cyclones, electrostatic precipitators, or other gas-solid separation systems.
During fluid bed, moving bed, and other types of surface retorting, decrepitation of oil shale occurs when particles of oil shale collide with each other or impinge against the walls of the retort forming substantial quantities of minute entrained particulates of shale dust. The use of hot spent shale as heat carrier material can aggravate the dust problem. Rapid retorting is desirable to minimize thermal cracking of valuable condensable hydrocarbons. Shale dust is also emitted and carried away with the effluent product stream during modified in situ retorting as a flame front passes through a fixed bed of rubblized shale, as well as in fixed bed surface retorting, but dust emission is not as aggravated as in other types of surface retorting.
Shale dust ranges in size from less than 1 micron to 1000 microns and is entrained and carried away with the effluent product stream. Because shale dust is so small, it cannot be effectively removed to commercially acceptable levels by conventional dedusting equipment.
The retorting, carbonization, or gasification of coal, peat, and lignite and the retorting or extraction of tar sands, gilsonite, and oil-containing diatomaceous earth create similar dust problems.
After retorting, the effluent product stream of liberated hydrocarbons and entrained dust is withdrawn from the retort through overhead lines and subsequently conveyed to a separator, such as a single or multiple-stage distillation column, quench tower, scrubbing cooler, or condenser, where it can be separated into fractions of light gases, light oils, middle oils, and heavy oils with the bottom heavy oil fraction containing essentially all of the dust. As much as 65% by weight of the bottom heavy oil fraction may consist of dust.
It is very desirable to upgrade the bottom heavy oil into more marketable products, such as light oils and middle oils, but because the heavy oil fraction is laden with dust, it is very viscous and cannot be pipelined. Dust-laden heavy oil plugs up hydrotreaters and catalytic crackers, abrades valves, heat exchangers, outlet orifices, pumps, and distillation towers, builds up insulative layers on heat-exchange surfaces reducing their efficiency, and fouls up other equipment. Furthermore, the dusty heavy oil erodes turbine blades and creates emission problems. Moreover, the dusty heavy oil cannot be refined with conventional equipment.
In an effort to solve this dust problem, electrostatic precipitators have been used as well as cyclones located both inside and outside the retort. Electrostatic precipitators and cyclones, however, must be operated at high temperatures and the product stream must be maintained at approximately the temperature attained during the retorting process to prevent any condensation and accumulation of dust on processing equipment. Maintaining the effluent stream at high temperatures allows detrimental side reactions, such as cracking, coking, and polymerization of the effluent product stream, which tend to decrease the yield and quality of condensable hydrocarbons.
Over the years, various processes and equipment have been suggested to decrease the dust concentration in the heavy oil fraction and/or upgrade the heavy oil into more marketable light oils and medium oils. Such prior art dedusting processes and equipment have included the use of cyclones, electrostatic precipitators, pebble beds, scrubbers, filters, electric treaters, spiral tubes, ebullated bed catalytic hydrotreaters, desalters, autoclave settling zones, sedimentation, gravity settling, percolation, hydrocloning, magnetic separation, electrical precipitation, stripping, and binding, as well as the use of diluents, solvents, and chemical additives before centrifuging. Typifying those prior art processes and equipment and related processes and equipment are those found in U.S. Pat. Nos. 1,668,898; 1,687,763; 1,703,192; 1,707,759; 1,788,515; 2,235,639; 2,524,859; 2,717,865; 2,719,114; 2,723,951; 2,793,104; 2,871,181; 2,879,224; 2,899,736; 2,904,499; 2,911,349; 2,952,620; 2,968,603; 2,982,701; 3,001,774; 3,008,894; 3,034,979; 3,058,903; 3,252,886; 3,255,104; 3,468,789; 3,519,552; 3,560,369; 3,684,699; 3,703,442; 3,784,462; 3,791,956; 3,799,855; 3,808,120; 3,900,389; 3,901,791; 3,910,834; 3,929,625; 3,951,771; 3,974,073; 3,990,885; 4,028,222; 4,040,958; 4,049,540; 4,057,490; 4,069,133; 4,075,080; 4,080,285; 4,082,043; 4,088,567; 4,094,770; 4,105,536; 4,132,030; 4,151,067; 4,151,073; 4,158,622; 4,159,949; 4,162,965; 4,166,441; 4,182,672; 4,199,432; 4,220,522; 4,225,420; 4,226,699; 4,242,102; 4,244,812; 4,246,093; 4,293,401; 4,312,747; 4,324,651; 4,350,582; 4,354,856; 4,388,179; 4,404,086; 4,415,430; 4,415,434; 4,415,442; 4,420,389; and 4,473,461 as well as in the articles by Rammler, R. W., "The Retorting of Coal, Oil Shale and Tar Sand By Means of Circulated Fine-Grained Heat Carriers as a Preliminary Stage in the Production of Synthetic Crude Oil", Volume 65, Number 4, Quarterly of the Colorado School of Mines, pages 141-167 (October 1970) and Schmalfeld, I. P., "The Use of The Lurgi/Ruhrgas Process For The Distillation of Oil Shale", Volume 70, Number 3, Quarterly of the Colorado School of Mines, pages 129-145 (July 1975). These prior art processes and equipment have not been successful to economically decrease the dust concentration in the heavy oil fraction to acceptable levels.
It is therefore desirable to provide an improved process for producing and dedusting synthetic oil.